1. Technical Field
The present invention relates to a tunable filter capable of separating wavelengths of light and method of manufacturing the tunable filter, and a sensing device.
2. Related Art
When light is multiple-reflected between a pair of mirrors (reflecting films) facing each other with a predetermined length of the gap, only light with a specific wavelength corresponding to the length of the gap is transmitted (wavelength-separated). Tunable filters are capable of selecting a wavelength band of the light to be transmitted by tuning the length of the gap. Various kinds of filters having structures, realized by micro-machining, for varying the length of the gap using electrostatic force (coulomb force) generated in accordance with a voltage applied between the pair of mirrors have been proposed.
Japanese Unexamined Patent Publication No. 2002-174721 and The U.S. Pat. No. 6,341,039 disclose examples of the filters having the structures described above.
The filter disclosed in Japanese Unexamined Patent Publication No. 2002-174721 is a filter formed by using the surface micro-machining technology, in which the gap is formed by removing a sacrifice layer by etching, the sacrifice layer being formed on a fixed mirror and a fixed electrode both formed on a substrate, and then being covered with a movable mirror and a movable electrode. Therefore, the length of the gap is determined by the thickness of the sacrifice layer, which causes a problem that the length of the gap is easy to be varied in accordance with the forming conditions of the sacrifice layer. If the length of the gap is varied, the coulomb force generated in accordance with a predetermined applied voltage is varied, thus it becomes difficult to arrange the length of the gap to be a desired value.
Incidentally, in the filter disclosed in The U.S. Pat. No. 6,341,039, the gap can be formed with precision because the gap is formed using a silicon dioxide (SiO2) layer, which is an embedded oxide film of a SOI (Silicon On Insulator) substrate, as the sacrifice layer. However, since the movable mirror is not insulated from the drive electrode equipped with the fixed mirror, a phenomenon called sticking may occur, in which the movable mirror is adhered to the drive electrode by the coulomb force.
Further, in either case of the examples, an opening (release hole) for introducing the etching fluid to remove the sacrifice layer is formed in, for example, the movable mirror formed on the upper surface of the sacrifice layer. Therefore, since the area of the electrode on the movable mirror is reduced as much as the area of the release hole, the drive voltage needs to be set higher in order for generating predetermined coulomb force, thus increasing the power consumption. Further, if the required length of the gap is small, the sticking caused by surface tension of water may occur in a cleaning process after removing the sacrifice layer. Therefore, a tunable filter having a structure that can be formed without removing the sacrifice layer is expected.
Further, the filters disclosed in the above patent documents have problem that the light to be separated is limited to infrared light because silicon is provided to the light transmitting paths.